1. Field of the Invention
The present invention relates to improvements in stereoscopic viewing devices and, more particularly, to a device for producing a three dimensional effect utilizing a single two dimensional image.
2. Summary of the Related Art
The existing devices for three dimensional viewing typically are divided into two broad categories, devices for projection of stereoscopic images and stereoscopic devices for viewing two dimensional images. For projection of stereoscopic images, such as in movie presentations, special stereo-diaprojectors or stereo overhead projectors may be used. Separation of the images is generally achieved through polarization filters in front of the projector objectives and through polarization spectacles worn by the user.
The present invention is primarily concerned with the second category, stereoscopic devices for viewing two dimensional images. Depth perception cues are modified or suppressed to alter the viewer's stereoscopic judgment. A pair of stereoscopic images is formed with lenses or mirrors positioned close to the eyes. The stereoscopic devices for viewing two dimensional images have been limited primarily to photographic slides and X-ray applications. The devices are bulky, expensive, and not generally suited for alternative uses, such as watching television.
In several more recent patents, devices have been disclosed which modify depth perception cues at the image such that viewers do not require individual devices to achieve three dimensional viewing. Considerable efforts have been made to create a simple and inexpensive means for providing stereoscopic television.
Three dimensional viewing, in general, relies upon two fundamental classes of depth perception cues: (1) binocular cues; and (2) pictorial cues.
The most generally acknowledged binocular cues are retinal disparity, convergence and accommodation. Retinal disparity refers to the disparity between the retinal image of an object in one eye as compared to the image of the same object in the other eye. Such disparity generally decreases as the square of the distance of the object and is interpreted by the mind as a function of distance. The depth effect produced in the mind by retinal disparity is known as stereopsis.
Convergence refers to the angle formed by the two eyes as they focus upon an object. The two eyes would be parallel if an object could be viewed at an infinite distance. Increased convergence seems to cause the mind to diminish the estimated size of the object.
Accommodation refers to the ability of the ciliary muscles surrounding the lens of an eye to alter the thickness of the lens, thereby sharply focusing the light rays coming from an object. Both convergence and accommodation are dependent upon the actual distance of the object from the observer and, therefore, in principle, could be utilized to judge absolute distance. However, in practice, the mind utilizes them only in the case of short distances. Accommodation by itself is operative only for approximately two or three feet. Convergence utilized by itself is a rather unreliable judge of distance. Utilized together, convergence and accommodation are operative to a distance of about ten feet from the observer.
Pictorial cues are those which are available if a scene is viewed with only one eye. Some of the most common pictorial cues are as follows:
interposition (partial covering of one object by another), PA1 shadow, PA1 linear perspective (convergence of lines as they recede into the distance), PA1 size perspective (visual angle of an object is inversely proportional to the distance), PA1 foreshortening perspective (differences in the projection of distances in the third dimension, e.g., differences in the sizes of railroad ties as the track recedes into the distance), PA1 detail perspective (loss of detail with distance), PA1 aerial perspective (distant objects become tinged with a blue color due to impurities in the atmosphere), PA1 texture gradients (the texture within a scene becomes more finely grained with distance), PA1 familiar size, and PA1 motion parallax (the diminution of apparent motion (whether of the observer or of the observed) with distance).
Cues may interact with one another and produce mental depth perception effects which the cues would not produce if operating in isolation from one another. Our limited knowledge, at the present time, of the human visual system and the interpretation of these cues by the human brain does not permit exact descriptions of the interactions of the various cues.
A number of devices have been developed for providing a three dimensional view from a two dimensional image. As noted above, one group of devices includes auxiliary optical means positioned close to the eyes, such as the devices disclosed in U.S. Pat. Nos. 2,084,795; 2,088,041; 2,297,322; 2,337,139; and 4,027,947. Another group of devices achieves a three dimensional effect by modifying the cues using devices positioned in the vicinity of the two dimensional image, such as the devices disclosed in U.S. Pat. Nos. 4,526,439; 4,651,219; and 4,756,601.
Although the patents referenced above do not expressly discuss the applicable cues, the patents may be interpreted as referring to devices which manipulate binocular cues (mainly retinal disparity and convergence) and, in some cases, pictorial cues.
U.S. Pat. No. 2,084,795 utilizes cylindrical lenses in close proximity to and entirely covering a two dimensional image, or cylindrical mirrors onto which a two dimensional image is projected, or a combination of the two. The fundamental purpose of the use of such lenses and mirrors is to alter the original two dimensional image in such a manner that the image seen by one eye has a different angle of rotation from that seen by the other eye. The explanation disclosed in the patent for the resulting three dimensional effect is "that when the right and left eyes are slightly rotated on their optical axes ... the resultant strain on the eye muscles is registered on the brain as distance." These optical manipulations involve the cues of retinal disparity or convergence. The present invention accomplishes this purpose by a very different means; namely, by means of small, flat mirrors positioned in proximity to the eyes of the observer.
Although there is little explanation as to how three dimensional viewing is achieved in U.S. Pat. Nos. 2,297,322; 2,337,139 and 4,027,947, the devices disclosed in the patents use a mirror to split a two dimensional image into two images: one of the images is a view of the two dimensional image from the left side and the other image is a view from the right side. Then the images are reversed so that the left eye views the right image and the right eye views the left image. Reversing the image alters the cues of retinal disparity or convergence.